1. Field of the Invention
The present invention relates to a current mirror circuit, and more specifically to a current mirror circuit suitable for a received signal indicator provided in a receiver for detecting a received electric field strength.
2. Description of Related Art
A receiver used in a communication system such as a PHS (personal handy-phone system) generally includes a received signal indicator for detecting a variation of a received electric field strength.
Referring to FIG. 1, there is shown a block diagram illustrating the construction of a conventional received signal indicator.
The shown received signal indicator, designated with Reference Numeral 102, is connected to a multi-stage amplifier 101 composed of a plurality of cascaded amplifiers for amplifying a received signal having an input power Pin. The received signal indicator 102 includes a detection circuit 103 for detecting an output power supplied from each of the amplifiers of the multi-stage amplifier 101, and a current mirror circuit 104 and a resistor R.sub.L for outputting, on the basis of an output of the detection circuit 103, a detection voltage V.sub.S in proportion to the input power Pin of the detection circuit 101. With this construction, a current Iref in proportion to the input power Pin of the detection circuit 101 is outputted from the detection circuit 103. Since the current mirror circuit 104 acting as a buffer amplifier is connected to the output of the detection circuit 103, an output current I.sub.O of the current mirror circuit 104 is caused to flow through the resistor R.sub.L. Thus, the detection voltage V.sub.S in proportion to the input power Pin of the detection circuit 101 is outputted from between opposite ends of the resistor R.sub.L.
Here, the current mirror circuit is a circuit operating to maintain a predetermined ratio between the input current Iref and the output current I.sub.O. Referring to FIG. 2, there is shown a circuit diagram of the simplest construction of the current mirror circuit, which is well known to persons skilled in the art.
In the circuit construction shown in FIG. 2, however, since a base current I.sub.B1 flowing between a base and an emitter of a transistor Q.sub.101 and a base current I.sub.B2 flowing between a base and an emitter of a transistor Q.sub.102 flows into the input current Iref, the output current I.sub.O becomes as follows: EQU I.sub.O =Iref+I.sub.B1 +I.sub.B2
In order to reduce the influence of the base currents, a current mirror circuit as shown in FIG. 3 has been proposed in the prior art.
The current mirror circuit shown in FIG. 3 includes a transistor Q.sub.111 having an emitter connected through a resistor R.sub.111 to a power supply voltage V.sub.CC, a transistor Q.sub.112 having a base connected to a base of the transistor Q.sub.111 and an emitter connected through a resistor R.sub.112 to the power supply voltage V.sub.CC, a transistor Q.sub.113 having a collector connected to the power supply voltage V.sub.CC and a base connected to a collector of the transistor Q.sub.111, a transistor Q.sub.114 having an emitter connected the bases of the transistors Q.sub.111 and Q.sub.112, a base connected to an emitter of the transistor Q.sub.113, and a collector connected to ground, and a constant current source 112 having one end connected to the emitter of the transistor Q.sub.113 and the other end connected to the ground.
Here, assume that a collector current of the transistor Q.sub.111 is I.sub.C1, a collector current of the transistor Q.sub.112 is I.sub.C2 (=I.sub.O), a base current of the transistor Q.sub.113 is I.sub.B3, an emitter current of the transistor Q.sub.114 is I.sub.E4, a base current of the transistor Q.sub.114 is I.sub.B4, and a current of the constant current source 112 is Ia. A relation between the output current I.sub.O and the input current Iref is expressed as follows: ##EQU1## where h.sub.FEP is a current amplification factor of the PNP transistors (Q.sub.111, Q.sub.112 and Q.sub.114) and h.sub.FEN is a current amplification factor of the NPN transistors (Q.sub.113).
As seen from the above equation (1), the prior art current mirror circuit shown in FIG. 3 has an error of Ia/(h.sub.FEN +1) between the input current Iref and the output current I.sub.O. However, since it is generally that h.sub.FEP, h.sub.FEN &gt;&gt;1, the error in the prior art current mirror circuit shown in FIG. 3 can be made smaller than that in the current mirror circuit shown in FIG. 2.
However, in the case that the prior art current mirror circuit shown in FIG. 3 is incorporated in the received signal indicator, since the value of the input current Iref varies in a logarithmic characteristics, the error becomes large when the value of the input current Iref becomes small. The reason for this is that, since the error of Ia/(h.sub.FEN +1) exists between the input current Iref and the output current I.sub.O as shown in the equation (1), the smaller the value of the input current Iref becomes, the larger the influence of the output current Ia (constant value) of the constant current source becomes non-negligibly. In addition, since the value of the current amplification factors h.sub.FEP and h.sub.FEN greatly varies dependently upon the manufacturing process, if the value of the current amplification factors h.sub.FEP and h.sub.FEN becomes small, the error between the input current Iref and the output current I.sub.O becomes large.